The field of the invention is gyromagnetic resonance spectroscopy, and particularly, nuclear magnetic resonance (NMR) techniques for measuring the properties of materials.
Gyromagnetic resonance spectroscopy is conducted to study nuclei that have magnetic moments and electrons which are in a paramagnetic state. The former is referred to in the art as nuclear magnetic resonance (NMR), and the latter is referred to as paramagnetic resonance (EPR) or electron spin resonance (ESR). There are other forms of gyromagnetic spectroscopy that are practiced less frequently, but are also included in the field of this invention.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.O) the individual magnetic moments of the paramagnetic nuclei in the tissue attempt to align with this field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented components in the perpendicular plane (x-y plane) cancel one another. If, however, the substance, or tissue, is irradiated with a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, can be rotated into the x-y plane to produce a net transverse magnetic moment M.sub.1 which is rotating in the x-y plane at the Larmor frequency.
The practical value of this gyromagnetic phenomena resides in the radio signal which is emitted after the excitation signal B.sub.1 is terminated. When the excitation signal is removed, an oscillating sine wave is induced in a receiving coil by the rotating field produced by the transverse magnetic moment M.sub.1. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.O, is determined by the magnitude of M.sub.1. The amplitude A of the emission signal (in simple systems) decays in an exponential fashion with time, t: EQU A=A.sub.Oe -t/T.sub.2
The decay constant 1/T.sub.2 is a characteristic of the process and it provides valuable information about the substance under study. The time constant T.sub.2 is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant, and it measures the rate at which the aligned precession of the nuclei dephase after removal of the excitation signal B.sub.1.
Other factors contribute to the amplitude of the free induction decay (FID) signal which is defined by the T.sub.2 spin-spin relaxation process. One of these is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. This is also called the longitudinal relaxation process as it describes the recovery of the net magnetic moment M to its equilibrium value M.sub.O along the axis of magnetic polarization (Z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances, and its independent measurement is the subject of many gyromagnetic procedures.
The measurements described above are called "pulse NMR measurements." They are divided into a period of excitation and a period of emission. As will be discussed in more detail below, this measurement cycle may be repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A variety of preparative excitation techniques are known which involve the application of one or more excitation pulses of varying duration. Such preparative excitation techniques are employed to "sensitize" the subsequently observed free induction decay signal (FID) to a particular phenomenon. Some of these excitation techniques are disclosed in U.S. Pat. Nos. 4,339,716; 4,345,207; 4,021,726 4,115,730 and 3,474,329.
Although NMR measurements are useful in many scientific and engineering fields, their potential use in the field of medicine is enormous. NMR measurements provide a contrast mechanism which is quite different from x-rays, and this enables differences between soft tissues to be observed with NMR which are completely indiscernible with x-rays. In addition, physiological differences can be observed with NMR measurements, whereas x-rays are limited primarily to anatomical studies.
For most medical applications utilizing NMR, an imaging technique must be employed to obtain gyromagnetic information at specific locations in the subject. The foremost NMR imaging technique is referred to as "zeugmatography" and was first proposed by P. C. Lauterbur in a publication "Image Formation by Induced Local Interactions: Examples Employing Nuclear Magnetic Resonance", Nature, Vol. 242, Mar. 16, 1973, pp. 190-191. Zeugmatography employs one or more additional magnetic fields which have the same direction as the polarizing field B.sub.O, but which have a nonzero gradient. By varying the strength (G) of these gradients, the net strength of the polarizing field B.sub.O =B.sub.z +G.sub.x X+G.sub.y Y+G.sub.z Z at any location can be varied. As a result, if the frequency response of the FID receiver coil and circuitry is narrowed to respond to a single frequency, .omega..sub.O, then gyromagnetic phenomena will be observed only at a location where the net polarizing field B.sub.O is of the proper strength to satisfy the Larmor equation; .omega..sub.O =.gamma. B.sub.O : where .omega..sub.O is the Larmor frequency at that location.
By "linking" the resulting free induction signal FID with the strengths of the gradients (G=G.sub.x, G.sub.y, G.sub.z) at the moment the signal is generated, the NMR signal is "tagged", or "sensitized", with position information. Such position sensitizing of the NMR signal enables an NMR image to be produced by a series of measurements. Such NMR imaging methods have been classified as point methods, line methods, plane methods and three dimensional methods. These are discussed, for example, by P. Mansfield and P. G. Morris in their book "NMR Imaging in Biomedicine" published in 1982 by Academic Press; New York.
The NMR scanners which implement these techniques are constructed in a variety of sizes. Small, specially designed machines, are employed to examine laboratory animals or to provide images of specific parts of the human body. On the other hand, "whole body" NMR scanners are sufficiently large to receive an entire human body and produce an image of any portion thereof.
In my U.S. Pat. No. 4,516,075 I describe an NMR scanner which produces images indicative of the motion of the gyromagnetic material. This is accomplished by a motion sensitizing step which is interlaced with the normal pulsed NMR measurement cycles. More specifically, a bipolar magnetic field gradient is applied to the gyromagnetic material after the excitation period, but before the subsequent FID signal. The direction of this motion sensitizing magnetic field gradient determines the flow direction in which the subsequent FID signal is sensitized, and the magnitude of the motion in that direction is determined by measuring the phase of the transverse magnetic moment M.sub.1. This phase is measured by detecting the component of the FID which is in phase with the excitation signal B.sub.1 and by detecting the component which is in quadrature with the excitation signal B.sub.1. These signals are digitized and processed along with position information to produce an image which is modulated in intensity as a function of the motion in the sensitized direction.
While this technique is very powerful and is very economical to implement in current NMR scanners, it is susceptible to visual "artifacts" caused by imperfections in the design or manufacture of the NMR scanner which produce phase errors at locations in the region of interest. For example, the excitation coils may introduce slight phase shifts in the excitation field B.sub.1 at specific locations throughout the region of interest. Such phase shifts are detected and processed to indicate motion where, in fact, none actually occurred. While such artifacts can be reduced by properly designing the NMR scanner and by filtering out the known errors, such measures increase the complexity and cost of the system.
In my co-pending U.S. patent application Ser. No. 760,354, now U.S. Pat. No. 4,654,591 entitled "NMR Flow Imaging Using Bi-Phasic Excitation Field Gradients" I describe a method and system for imaging motion which is insensitive to phase errors produced by a non-homogeneous excitation field. This technique requires the application of excitation field gradients, and this in turn requires separate excitation field gradient coils and associated drive circuits. These add to the cost of the instrument.